Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink can be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. The printer supplies either aqueous liquid ink or a phase change ink in a liquid phase to printheads for ejection through inkjets onto an image receiving surface of an image receiving member, such as a print medium or an indirect imaging belt or imaging drum. Liquid inks dry and phase change inks cool into a solid state after being transferred to a print medium, such as paper or any other suitable medium for printing.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving member to form an ink image. The image receiving member can be a continuous web of recording media, a series of media sheets, or the image receiving member can be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, or electrostatic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify the inkjets in the printheads of the printer that must be operated to eject a pattern of ink drops at particular locations on the image receiving member to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, an imaging operation can be viewed as the placement of ink drops on an image receiving member with reference to electronic image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads are registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads refers to a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the relative positions of the printheads with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with one another across a width of the image receiving member and that all of the inkjets in the printhead are operational. The presumptions regarding the positions of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads to better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
During operation, individual inkjets in the printheads eject patterns of ink drops to form printed images, including text and graphics, on the image receiving surface. An individual inkjet includes a fluid pressure chamber that holds ink prior to ejecting each ink drop and a larger ink reservoir replenishes the pressure chamber after the ejection of each ink drop. When printing patterns of multiple ink drops during a print job, the transient motion of ink may result in variations of the mass and velocity of the ink drops that are ejected from the inkjet. Since the printhead is located at a substantially fixed distance from the moving image receiving surface, the variations in the ink drop velocity also affect the locations of where the ink drops land on the image receiving surface. The variations can lead to errors in the placement of ink drops that degrade the quality of the printed image.
Because the variations in the ink drop masses and velocities vary over time based on the pattern of operation for the inkjet, traditional registration processes are not suitable for correcting the drop placement errors. In many printer embodiments, the electrical firing signals that operate inkjets in a printhead are generated in a synchronous manner based on a clock signal that is generated at a predetermined frequency. During each period of the clock signal, the inkjet either receives the electrical firing signal to eject an ink drop, or does not receive the electrical firing signal and does not eject an ink drop. One existing solution that adjusts the relative locations of ink drops from a single inkjet adjusts the time of generation for the electrical firing signals forward or backward in time by one or more cycles of the clock signal. Commonly owned U.S. Pat. No. 8,004,714 describes a process for modifying image data to adjust the timing for generation of firing signals for ink drops by one or more cycles of the clock signal to correct for ink drop placement errors for different patterns of ink drops that the inkjet ejects during operation.
While the existing solutions for drop placement adjustment correct for some drop placement errors due to variations in the velocity of the ink drops, other drop placement errors are not well suited to correction by adjusting the time of ink drop ejection. For example, in some printed ink drop patterns, changing the clock cycle during which the inkjet ejects an ink drop includes selecting a clock cycle when the inkjet is already scheduled to eject an ink drop during a print job. Thus, the existing techniques would either print only one ink drop when the image data specify that two ink drops should be printed, or the inkjet ejects two ink drops, but at least one ink drop is not ejected during the optimal clock cycle to correct the position error. Another drawback of the existing correction process is that the printer is only capable of adjusting the time for ejection of the ink drops by an integer number of cycles in the clock signal. In some instances, the position error for the printed ink drop lies within the distance that the image receiving surface moves during a full cycle of the clock signal. Thus, changing the clock cycle during which an ink drop is ejected, which is referred to as a full-pixel adjustment, cannot compensate for sub-pixel errors that are not aligned with full-pixel intervals of the image receiving surface. Consequently, improved systems and methods for the operation of inkjets to reduce drop placement errors while printing patterns of ink drops would be beneficial.